1. Field:
The invention is in the field of production and harvesting of algae, together with their associated bacteria.
2. State of the Art:
Algae are single cell microorganisms, sometimes called algal cells. There are many different types of algae, some of which are commonly known by such layman terms as "pond scums", "frog spittle", "water mosses", "seaweeds", etc. Some algae grow in fresh water and some in brackish water. Some, such as Dunaliella teodoresco (usually called just Dunaliella) will even thrive in extremely salty water, e.g. the Dead Sea in Israel, the Huth lagoon in Australia, the Searle's Lake region in California, and the Great Salt Lake in Utah. There are several strains of Dunaliella, e.g. Dunaliella salina, Dunaliella bardawill, and others. However, these all have quite similar properties. Algae, even the Dunaliella, are a rich source of food for fish and animals, and are also a source of many other valuable products, such as oil, glycerol, beta-carotene, and others. Consequently, the production and harvesting of algae constitutes an important industry.
In common with practically all living organisms, the various forms of algae also have their predators. Dunaliella has an advantage in that it can thrive in a highly concentrated saline medium wherein few of its predators can survive. Thus, in such a medium, it can grow practically unharmed by predators. In addition the high salt concentration has another advantage in that it considerably increases the production of the valuable b-carotene.
Dunaliella can also exist in saline waters having a low concentration of salt. Thus, in shallow pools or ponds a dilution of the salt content by rainwater or melting snow will not destroy the algae. Because of this ability to thrive in saline waters having a wide range of salt content, it is not necessary to closely control the salt concentration level.
Dunaliella also has a weakness, in that, unlike many other algae, it does not have a strong and rigid outer shell. Thus, it can be easily damaged by mishandling.
Due to its great potential for providing useful products, many schemes have been devised in an attempt to improve the methods of growing and harvesting dunaliella and of extracting the useful products therefrom. This algae and the bacteria associated with it normally grow in a saline liquid medium from which it must be separated. Some prior methods for separating the Dunaliella from the liquid medium have utilized filters. However, the Dunaliella algae are quite small, being ovoids of approximately 25.times.8-15 micron size, and the bacteria are mostly smaller than five microns. Due to the lack of a rigid cell wall, the cells are quite flexible and can pass through filters as small as five microns. Also, they are quite slimy. The result is that filters, having a small enough mesh size to effectively filter, rapidly become clogged and necessitate a frequent and very difficult cleaning operation. Filter aids, such as diatomaceous earth, have been employed. However, the use of these results in a cake that must be removed and further processed.
Some experimenters have utilized a centrifuge to effect separation. However, since the Dunaliella cells are in a saline solution that usually has a high specific gravity and, since they are substantially neutrally buoyant, centrifuging requires a very high energy input. In addition, centrifuging damages the cells by breaking their outer membranes, resulting in a loss of desirable constituents.
In some cases, both filtration and centrifuging have been augmented by high pressure to increase their effectiveness. However, this results in still greater damage to the tender Dunaliella cells, leading to still further loss of important constituents, such as glycerol and carotenoids.
Chemical separation has also been proposed, but apparently has not been cost effective. However, one such method that has recently been devised comprises floculating the Dunaliella algae with chemical additives. This procedure may prove useful as one step in the harvesting operation of the invention, but by itself would be too costly since large quantities of chemicals would be needed to accomplish floculation in a large volume of water.
More recently, as disclosed by Kessler, U.S. Pat. No. 4,324,067 of Apr. 13, 1982, the use of fibrous material extending down into the liquid medium has been employed to effect the migration of the algal cells to a harvesting zone above the surface of such medium. This, however, entails the use of a large auxiliary structure covering essentially the entire surface area of the growth reservoir. The provision of such a structure is very undesirable from the standpoints of cost, maintenance, light blockage, and the need for protection against the elements.
In summary it should be noted that experience and research has shown that, even under essentially ideal conditions, only about twenty-five grams of Dunaliella algae will grow per cubic meter of growth medium. This is the natural result of conversion of sunlight energy to compounds with stored energy by plants such as algae, and necessitates the utilization of large bodies of water having extensive surface areas to produce large quantities of the algae.
It is evident that there is a real need for an improved process, system, and apparatus for growing and harvesting the algae as a biomass suitable for further processing.